So I'm not sure what is going on? I'm currently subscribed to 533 threads.

That's quite a number.

We constantly monitor our mail server's performances and currently there is no message of abuse of network resources of any kind.

However, according to our logs, your provider decided unilaterally to periodically bounce mails from our mail server after a certain threshold of incoming mails is reached. Perhaps the amount of mails is triggering some internal spam check score; unfortunately, they are using hard bounces, meaning, the mail is not going to be resent. I have filled out a request for removal of the bounce.

Update: According to our logs, your provider has put the whole /24 subnet on their bounce list. Maybe someone in this network did send spam in the past, but certainly it wasn't us. Will await a response from them.